(1) Field of the Invention
This invention relates to a method for selective etching of titanium nitride layers useful in fabrication of semiconductor integrated circuits, and more particularly to subtractive etching of titanium nitride by reactive plasma etching, and still more particularly to subtractive etching of titanium nitride layers in a plasma reactor system particularly configured for a specific range of plasma reactive etch processes.
(2) Description of the Prior Art
In the fabrication of semiconductor integrated circuits, layers of refractory metal nitride compounds and particularly titanium nitride are used for the purpose of passivation and to provide an antireflection coating on top of previously deposited layers used in the circuit. Such layers of titanium nitride must themselves be formed into useful patterns by selective etching, and this is done conventionally by subtractive etching through a photoresist pattern, using a chemically reactive plasma to bring about the desired subtractive chemical reactions. In such a process, a reactive gas or mixtures of reactive gases such as trifluoromethane CHF3 or tetrafluoromethane CF4 are excited in a reaction chamber by an electrical plasma sustained therein by radio # frequency energy input to provide the reactive species which react with the titanium nitride layer to bring about the subtractive etching. The entire process of plasma activation of the gas and subtractive etching is normally carried out in a plasma reactor chamber specifically designed and operated for this purpose.
It is customary in manufacturing of integrated circuits to employ various systems for reactive etching which are optimally designed and operated for narrowly specified conditions and results. It is commonly observed that a system so narrowly attuned to specific materials and process objectives cannot readily be employed for widely different materials or substantially altered process objectives. Thus, for example, a particular system widely employed for reactive plasma etching of various materials known as the LAM 590 is supplied by LAM Systems Inc., Fremont, Calif., USA. This system, while useful in circuit manufacturing for conventional subtractive etching of passivation layers of silicon oxide and the like, is generally not considered suitable for plasma -enhanced etching of titanium nitride in gas mixtures of CHF3 and CF4 within the normal operating limits of the system, due to difficulties with electrical arcing, resputtering of material, and electrical damage to integrated circuits due to excessive ion bombardment from the ions in the plasma.
In the conventional integrated circuit fabrication process for forming an electrical contact to a conductive layer covered first by an antireflection coating layer of, for example, titanium nitride and then by an insulating passivation layer of, for example, silicon oxide or silicon nitride, subtractive etching of the passivation layer must first be done selectively to begin formation of the electrical contact via hole. This must then be followed by subtractive etching of the titanium nitride layer to complete the formation of the electrical contact via hole. The first subtractive etching step, of the passivation layer, may be done conventionally in a plasma reactor system such as a LAM 590 system previously mentioned. However, the second subtractive etching step of the titanium nitride layer is normally then done in a different plasma reactor system. This is because the system parameters of the LAM 590 plasma reactor system set up for pasivation layer etching will result in problems such as electrical arcing and discharge, excessive ionic bombardment of the integrated circuit, and resputtering and redistribution of material when attempt is made to subtractively etch titanium nitride layers. Such difficulties would result in lower yield and greater cost in manufacturing as well as reduced reliability of integrated circuit devices, and therefor the need fora second system results in increased cost in the manufacturing of integrated circuit devices.
It is known in the prior art that titanium compounds such as titanium nitride can be selectively etched by active halogen species generated in an electrical plasma. U.S. Pat. No. 5,326,427 to Jerbic describes the etching of titanium-containng materials by chlorine or fluorine atoms obtained from an electrical discharge in molecular chlorine or fluorine-containing gases. U.S. Pat. No. 5,419,805 to Jolly describes the etching of titanium nitride by reactive halogen species obtained from an electrical plasma sustained in a mixture of CF4 and C2F6 gases. Neither patent describes the use of a specific plasma reactor design, system configuration, or process description to bring about the desired subtractive etching reactions, nor is there any discussion of the use of commercially-available standard plasma reactor equipment for this purpose.